Why Load Testing?

Load testing reveals how your system behaves under stress. It's essential for capacity planning, finding bottlenecks, and preventing production incidents.

Types of Load Tests

Load Test

Simulate expected peak load:

// k6 load test
export const options = {
  stages: [
    { duration: '2m', target: 100 },  // Ramp up
    { duration: '5m', target: 100 },  // Stay at peak
    { duration: '2m', target: 0 },    // Ramp down
  ],
};

Stress Test

Find the breaking point:

export const options = {
  stages: [
    { duration: '2m', target: 100 },
    { duration: '5m', target: 100 },
    { duration: '2m', target: 200 },
    { duration: '5m', target: 200 },
    { duration: '2m', target: 300 },  // Keep increasing
    { duration: '5m', target: 300 },
  ],
};

Spike Test

Simulate sudden traffic surge:

export const options = {
  stages: [
    { duration: '10s', target: 100 },
    { duration: '1m', target: 100 },
    { duration: '10s', target: 1000 }, // Spike!
    { duration: '3m', target: 1000 },
    { duration: '10s', target: 100 },
  ],
};

Soak Test

Test for memory leaks and degradation:

export const options = {
  stages: [
    { duration: '5m', target: 100 },
    { duration: '8h', target: 100 },  // Run for hours
    { duration: '5m', target: 0 },
  ],
};

Writing Realistic Tests

k6 Test Script

import http from 'k6/http';
import { check, sleep } from 'k6';

export default function() {
  // Simulate user journey
  const homeRes = http.get('https://api.example.com/');
  check(homeRes, {
    'home status 200': (r) => r.status === 200,
    'home response time < 500ms': (r) => r.timings.duration < 500,
  });
  
  sleep(1); // Think time
  
  const loginRes = http.post('https://api.example.com/login', {
    email: 'test@example.com',
    password: 'password',
  });
  check(loginRes, {
    'login successful': (r) => r.status === 200,
  });
  
  const token = loginRes.json('token');
  
  // Authenticated request
  const profileRes = http.get('https://api.example.com/profile', {
    headers: { Authorization: `Bearer ${token}` },
  });
  
  sleep(Math.random() * 3); // Variable think time
}

Key Metrics

What to Measure

• Response Time: p50, p95, p99
• Throughput: Requests per second
• Error Rate: Failed requests percentage
• Concurrent Users: Active virtual users

Thresholds

export const options = {
  thresholds: {
    http_req_duration: ['p(95)<500'],  // 95% under 500ms
    http_req_failed: ['rate<0.01'],    // Less than 1% errors
    http_reqs: ['rate>100'],           // At least 100 RPS
  },
};

CI/CD Integration

# GitHub Actions
jobs:
  load-test:
    runs-on: ubuntu-latest
    steps:
      - uses: grafana/k6-action@v0.3.0
        with:
          filename: load-tests/api-test.js
          flags: --out cloud

Analyzing Results

• Compare against baseline
• Identify bottlenecks (CPU, memory, database)
• Correlate with application metrics
• Document findings and recommendations

Conclusion

Load testing should be part of your regular development cycle. Test early, test often, and always test before major releases.